In answer to the question, yes, we could say that neutrinos travel in a straight line from the source in which they were produced. In fact, it is what makes them attractive, among other things, for astrophysics. That they do not deviate from the place where they were generated gives us information about where the source is located. If we are able to detect the neutrinos produced during a supernova explosion, we could know where that supernova occurred. This would be the short answer, but it is necessary to explain it in detail because it is not that simple.
Neutrinos are fundamental particles, without electrical charge and very small—so small that for a time it was thought that they had no mass—so they interact weakly with matter. During nuclear reactions inside stars, different particles such as photons or neutrinos are produced. Unlike photons—which, although they have no charge or mass, behave as if they did when interacting with matter—neutrinos escape following a rectilinear path without interacting with it.
That happens inside a star, but what happens in the universe? In the cosmos, neutrinos move the same. As they have no charge, they do not interact with electromagnetic fields that could divert them from their path, and it is also very unlikely that they will interact with matter due to their low mass. That is why we say that neutrinos travel in straight lines, because they do not interact with the rest of the matter in the universe. This does happen to other particles, such as cosmic rays, which interact with electromagnetic fields or photons.
Neutrinos could interact with matter through the force of gravity. However, although they have mass, it is so small that even in the event that you encounter a very massive object that significantly curves space-time, such as a supermassive black hole, the probability of interaction is practically negligible.
However, we must clarify that they move in a straight line because the space-time of the universe is not flat, but curved due to the matter it contains. Taking this curvature into account, neutrinos actually move along geodesic lines (which are the shortest path joining two points on a given surface). It is something analogous to what happens with the trajectory of airplanes on the surface of the Earth: they do not follow straight lines between, for example, Madrid and New York, but they traditionally deviate towards the north when crossing the Atlantic Ocean due to the curvature of the Earth. Of course, if we limit ourselves to a small region of the universe without large masses, what is known as Minkowski space, the curvature is minimal and the geodesic line coincides with a rectilinear path.
In summary, to answer this question, I would say that neutrinos move along geodesic lines, which in the absence of large masses that significantly curve space-time, such as supermassive black holes, coincide with straight-line trajectories.
And this straight-line movement of neutrinos is precisely one of the properties that makes them so interesting for astronomy. That and they are very fast particles. They are believed to travel at speeds very close to that of light. In fact, some studies argued that they could overcome it, although the results were highly disputed. Because they are so fast and there is nothing to deflect them, they reach Earth before the photons. And that, for example, detecting a supernova explosion is very important. The people who do research with neutrino detectors—such as Super-Kamiokande, located in Japan and where I work—are prepared so that, in the event of a supernova explosion, they can launch an alert in a matter of seconds. It is only necessary to detect the neutrinos and tell the telescopes, both terrestrial and space, where they should point to observe the phenomenon from the beginning of the explosion.
Nataly Ospina Escobar She is a researcher in the Department of Theoretical Physics of the Autonomous University of Madrid.
Question sent via email by Ada Cobo.
Coordination and writing: Victoria Toro.
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